Tuesday, February 22, 2011

Severe weather for the south-central US

A lot is happening in the weather across the country over the next few days. Today I'm going to focus on the severe weather threat Thursday into Thursday night for the south-central part of the country. Tomorrow I'm going to talk about the snow event that is shaping up for Wednesday night into Thursday here in Seattle.

The shortwave trough that moved through a week or so ago that initially spawned some severe weather hype passed through without much of a fuss in terms of severe weather--big snow event, though. This shortwave moving through this week, however, has a bit more certainty to it with regards to it being a severe weather producer. So what's different about this week?

Right now we have pretty zonal (west-east) flow across the eastern half of the country with a bit of a ridge building over the Great Lakes. However, two troughs are approaching from the west. One is somewhat off the map but just entering the Pacific northwest (more on that one tomorrow). The second is the cut-off low height center off the southern California coast. This cut-off low is forecast to be picked up into the main west-to-east flow and move into the southwestern US tonight and tomorrow. By Thursday morning, the GFS shows this at 500mb:

Notice that there seem to be two main jet streams in the forecast map above--one involves jet streaks oriented west to east across the northern tier of the country. However, another is oriented southwest to northeast stretching from northern Mexico into the midwest. The presence of two jet streams like this is important--it makes this a potential severe thunderstorm event instead of another snowstorm. Why is this?

During much of the winter, we typically see only one major jet stream across the US--the polar jet. Remember that strong winds aloft form as a response to temperature gradients below. As such, this one polar jet lies over the "polar front"--the boundary separating frigid arctic air from warmer, moister subtropical air.

Fig 3 -- Schematic of the single polar jet.

As warm, moist air is advected northward to the east of a low pressure center, it moves into areas that were previously underneath frigid arctic air. This not only limits instability (we'd want it to be really warm at the surface and cold aloft to get good instability) but also causes the warm air to cool off considerably as it heads north. As long as you are north of the polar jet, you're usually having very cold weather.

So what happens when the polar jet moves further north, particularly as we start warming up during the spring and into summer? It leaves behind a somewhat cooler (but not frigid) air mass over the country. However we also still get warm, moist, subtropical air trying to intrude north. This time, however, it's moving into a less-cold air mass. Furthermore, there's still a boundary between the cool air over the continent and the frigid air up north underneath the polar jet. So we have two areas of temperature gradients--one between the frigid air up north and the cool air over the central continent and another between that same cool air over the central continent and warm, moist air coming from the Gulf of Mexico. With two temperature gradients, we see two jets:

Fig 4 -- Typical spring time pattern with the polar jet further north and the "subtropical" jet to the south.

The southern jet is typically called the "subtropical jet", though usually not until it has become more organized later in the year. Why is this setup better for severe weather? Instead of the warm, moist semi-tropical air moving into frigid arctic air, it's moving into a moderated air mass that's typically cool and very dry. Since the middle part of the country has warmed up some (as compared to when it was under the arctic air), it's much easier to warm the surface enough to generate strong instability. It also means that frontally-generated precipitation can usually fall as rain instead of us worrying about temperatures near the surface being cold enough for snow.

So we have a jet pattern similar to this forecast for this Thursday morning. Does the surface temperature profile match this model? It certainly does.

The cold, arctic air is confined to the northern part of the country--north of the polar jet. There are cool temperatures in a region extending from the central plains through the upper midwest, and the warm, moist tropical air is located in the southern plains and lower Mississippi valley. We can see a surface low also forming in west Texas underneath the divergent exit region of a jet streak in the subtropical jet stream (compare it to figure 2 above). We still see strong thermal contrasts around the developing surface low, indicating a cold front stretching south and a warm front stretching east. However, the frigid arctic air we would be expecting for a winter storm is left well to the north.

Is there moisture in that expanding warm sector across the southern plains? The GFS seems to think there will be:

This forecast sounding shows strong directional wind shear from southerly winds at the surface to southwesterly winds aloft--a good directional shear for severe storms. There's also speed shear--from 10 knots at the surface to 40 knots at 10,000 feet. With this combination, the hodograph has a nice, clockwise curvature that we like to see for severe weather. The lapse rate isn't extremely steep--in fact it looks close to moist adiabatic for much of the way up. Correspondingly, only 120 J/kg of surface-based CAPE is being forecast for that sounding. This is something to watch--we'd want stronger instability for very severe storms. However, the lift provided by the front should be enough to fire off some storms. This is also just a sample sounding at one time at one point--instability can be greater elsewhere. In fact, the GFS SB-CAPE forecast map looks like this:

The best CAPE values are forecast to pool along the frontal boundary from southeastern Oklahoma down through Texas--and even then they aren't terribly impressive. There's also a small area out in front of the front in southern Arkansas and northern Louisiana where the instability seems to be enhanced. This area would probably see the best chance for tornadic storms--with wind shear like what we saw in that forecast sounding, there's definitely the potential for rotating storms. However, if the front becomes the main lifting mechanism, this will probably reduce to a hail-wind event, at least in the storms along the front.

The OWL-WRF model does fire some storms out ahead of the main front, but the structure seems very unorganized:

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Completed graduate school at the University of Washington, now a postdoctoral researcher at NCAR in Boulder. The thoughts and opinions expressed on this blog are solely those of the author and are do not necessarily represent the positions of NCAR.
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